In a gas and steam turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is utilized for the generation of steam for the steam turbine. Heat transfer takes place in a waste-heat steam generator disposed downstream of the gas turbine and in which a number of heating surfaces for water preheating, steam generation and steam superheating are normally disposed. The heating surfaces are connected into the water/steam circuit of the steam turbine. The water/steam circuit normally contains several, e.g. three, pressure stages, in which case each pressure stage may have an evaporator heating surface.
For the steam generator mounted downstream of the gas turbine on the heating-gas side as a waste-heat steam generator, a number of alternative design concepts are suitable, namely configuration as a continuous steam generator or as a circulation steam generator. In the case of a continuous steam generator, the heating of steam-generator tubes provided as evaporator tubes results in evaporation of the flow medium in the steam generator tubes in a single pass. In contrast, in the case of a natural or forced circulation steam generator, the circulating water is only partly evaporated as it passes through the evaporator tubes, the water that is not evaporated being is re-fed to the same evaporator tubes for further evaporation after separation of the generated steam.
A continuous steam generator, in contrast to a natural or forced circulation steam generator, is not subject to any pressure limitation, which means that live-steam pressures well above the critical pressure of water (Pcri≈221 bar)—where there is only a slight difference in density between a fluid-like medium and a steam-like medium—are possible. A high live steam pressure promotes high thermal efficiency and therefore low CO2 emissions from a fossil-fired power plant. In addition, a continuous steam generator has a simple type of construction compared with a circulation steam generator and can therefore be manufactured particularly inexpensively. The use of a steam generator designed according to the continuous principle as the waste-heat steam generator of a gas and steam turbine plant is therefore particularly advantageous for achieving a high overall efficiency of the gas and steam turbine plant using a simple type of construction.
Particular advantages in terms of manufacturing costs, but also of maintenance required, are provided by a horizontally constructed waste-heat steam generator in which the heating medium or heating gas, i.e. the exhaust gas from the gas turbine, is passed through the steam generator in an approximately horizontal flow direction. However, with a horizontally constructed continuous steam generator the steam generator tubes of a heating surface may be subjected to markedly differential heating depending on their positioning. Particularly in the case of steam generator tubes connected to a common header on the output side, differential heating of individual steam generator tubes may result in a combining of steam flows with greatly differing steam parameters and therefore undesirable efficiency losses, in particular comparatively diminished effectiveness of the heating surfaces affected and consequently reduced steam generation. Differential heating of adjacent steam generator tubes may also result in damage to the steam generator tubes or the header, particularly in the region where they discharge into headers. The per se desirable use of a horizontally constructed continuous steam generator as a waste-heat steam generator for a gas turbine can therefore entail considerable problems in terms of sufficiently stabilized flow control.
EP 0 944 801 B1 discloses a steam generator suitable for a horizontal type of construction and additionally having the abovementioned advantages of a continuous steam generator. To this end, the disclosed steam generator is designed in respect of its continuous evaporator heating surface in such a way that a steam generator tube heated more than another steam generator tube of the same continuous evaporator heating surface has a higher throughput of flow medium than the other steam generator tube. If differential heating of individual steam generator tubes occurs, the continuous evaporator heating surface of the steam generator disclosed therefore exhibits, in its flow characteristic typical of a natural circulation evaporator heating surface (natural circulation characteristic), a self-stabilizing behavior resulting in a matching of the outlet-side temperatures even to differentially heated steam generator tubes connected in parallel on the flow medium side without the need for external intervention. However, this concept requires that the disclosed steam generator be designed for feeding with flow medium having comparatively low mass flow density.